Optical element, method for manufacturing optical element and semiconductor laser device using the optical element

ABSTRACT

The present invention provides an optical element which can reliably acquire a difference of refractive indices between a member under a photonic crystal layer and the crystal layer without using such a stacking technique as in conventional processes; a method for manufacturing the optical element; and a semiconductor laser device with the use of the optical element. The optical element has the first layer  500  and the second layer  400  formed on a substrate  100 , wherein the second layer includes pores and has a refractive-index periodically changing structure in which a refractive index periodically changes in an in-plane direction; and the first layer has an oxidized region with a lower refractive index than the refractive index of the second layer, in a lower side of the pores of the second layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical element, a method formanufacturing the optical element and a semiconductor laser device usingthe optical element.

2. Description of the Related Art

In recent years, research on a photonic crystal has been activelyconducted.

The photonic crystal means a structure which is formed of substancesperiodically repeating different refractive indices, and thereby causesa wavelength range (photonic band gap) in which light is inhibited frompropagating, similarly to a band gap existing in an electronic state ofa crystal.

By using the photonic crystal, it becomes possible to two-dimensionallyor three-dimensionally confine light in the crystal, so that thephotonic crystal is tried to be applied to an optical waveguide or amirror for a semiconductor laser.

Technologies with the use of various techniques have been proposed inorder to produce the two-dimensional and three-dimensional dielectricswith the periodic structure for the photonic crystal. However, thephotonic crystal having the two-dimensional periodic structure is morelikely to be realized because the structure can be produced with alithographic technology.

Incidentally, an ideal two-dimensional photonic crystal shall have auniform and infinite length in a Z-axis direction, but it is practicallyimpossible to produce such a crystal. In addition, when thetwo-dimensional photonic crystal is long in the Z-axis direction (filmthickness direction), light inevitably spreads toward outside of theZ-axis direction. Accordingly, when the two-dimensional photonic crystalis imparted a function such as amplification and absorption, the lengthbrings disadvantage.

For this reason, when the two-dimensional photonic crystal is actuallyused, it needs to confine the light which propagates in the Z-axisdirection, by any means.

Specifically, it is necessary to make a difference between refractiveindices of a photonic crystal layer placed on a substrate and thesubstrate large.

In order to meet such a need, for instance, Japanese Patent ApplicationLaid-Open No. 2000-232258 (Patent Document 1) discloses a technology inFIG. 2 for realizing a slab-type photonic crystal formed of two layershaving a large difference of a refractive index between them bycombining a semiconductor layer with a dielectric layer while using astacking technique.

SUMMARY OF THE INVENTION

However, in order to manufacture a slab-type photonic crystal disclosedin Patent Document 1, a high grade of process technology is necessaryfor the stacking step.

The high grade of the process technology is required specificallybecause a necessary layer in the stacking step needs to be removedafterwards because the layer is unnecessary for the element, and oneepitaxial substrate alone cannot realize the element, which make theprocess complicated.

The present invention is designed with respect to the above describedproblem and is directed at providing an optical element which canreliably acquire a difference of refractive indices between a memberunder a photonic crystal layer and the crystal layer without using sucha stacking technique as in conventional processes; a method formanufacturing the optical element; and a semiconductor laser device withthe use of the optical element.

In order to achieve the above described object, the present inventionprovides an optical element having the configuration described below; amethod for manufacturing the optical element; and a semiconductor laserdevice with the use of the optical element.

The present invention is directed to an optical element having a firstlayer and a second layer formed on a substrate, wherein the second layerincludes pores and has a refractive-index periodically changingstructure in which a refractive index changes periodically in anin-plane direction; and the first layer has an oxidized region with alower refractive index than the refractive index of the second layer, ina lower side of the pores of the second layer.

In the optical element, the first layer can include Al.

In the optical element, the difference of refractive indices between theoxidized region and the second layer can be 1.0 or more.

In the optical element, the first layer and the second layer can includeAl, Ga and As.

In the optical element, the first layer can comprise the oxidized regionand an unoxidized region.

The present invention is directed to a semiconductor laser devicecomprising the optical element, wherein the optical element is used as amirror in a surface emitting laser.

The present invention is directed to a method for manufacturing anoptical element, comprising the steps of: preparing a member having afirst layer on a substrate, and having in the upper part of the firstlayer a second layer that contains pores and has a refractive-indexperiodically changing structure in which a refractive index changesperiodically in an in-plane direction; and then secondly forming anoxidized region having a lower refractive index than the refractiveindex of the second layer, by oxidizing a part of the first layerlocated in a lower part of the pores.

In the method for manufacturing an optical element, the first layer andthe second layer can be formed from a compound semiconductor containingAl, and a ratio of Al contained in the first layer is higher than aratio of Al contained in the second layer.

The method for manufacturing an optical element can comprise a step ofremoving the oxidized region from the first layer after the step offorming the oxidized region.

The present invention is directed to a method for manufacturing areflecting mirror to be used in a vertical cavity surface emittinglaser, comprising the steps of: preparing a member having the firstlayer stacked on a substrate and the second layer stacked on the firstlayer; arraying in the second layer a plurality of pores each of whichextends to a stacked direction periodically in the in-plane directionwith respect to the substrate; and

oxidizing a part of the first layer located in a lower part of the poresto increase a difference of refractive indices between the first layerand the second layer.

The present invention can realize an optical element which can reliablyacquire a difference of refractive indices between a member under aphotonic crystal layer and the crystal layer without using such astacking technique as in conventional processes; a method formanufacturing the optical element; and a semiconductor laser device withthe use of the optical element.

In other words, the present invention can form a slab-type photoniccrystal with a large difference of refractive index even in asemiconductor optical element having a plurality of layers withdifferent compositions, in an easy process; and also can provide aslab-type two-dimensional photonic crystal element which can be appliednot only to a passive element but also to an active element such as alaser.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C are schematic views for describing a slab-typetwo-dimensional photonic crystal element in Example 1 according to thepresent invention.

FIGS. 2A, 2B, 2C and 2D are views for describing a slab-typetwo-dimensional photonic crystal element in Example 1 according to thepresent invention, in which FIGS. 2A and 2C are views for describing thecase of producing a waveguide, and FIGS. 2B and 2D are views fordescribing the case of producing a resonator.

FIGS. 3A, 3B, 3C, 3D and 3E are views for describing a method formanufacturing a slab-type two-dimensional photonic crystal element inExample 1 according to the present invention.

FIG. 4 is a schematic view for describing a surface emitting laser withthe use of a slab-type two-dimensional photonic crystal element inExample 2 according to the present invention.

FIGS. 5A, 5B, 5C, 5D, 5E, 5F, 5G, 5H and 5I are views for describing amethod for manufacturing a surface emitting laser with the use of aslab-type two-dimensional photonic crystal element in Example 2according to the present invention.

FIG. 6 is a schematic view for describing a surface emitting laser withthe use of a slab-type two-dimensional photonic crystal element inExample 3 according to the present invention.

FIGS. 7A, 7B, 7C, 7D, 7E, 7F, 7G, 7H and 7I are views for describing amethod for manufacturing a surface emitting laser with the use of aslab-type two-dimensional photonic crystal element in Example 3according to the present invention.

FIGS. 8A, 8B and 8C are schematic views for describing a slab-typetwo-dimensional photonic crystal element provided with an air-bridgestructure in Example 4 according to the present invention.

FIGS. 9A, 9B, 9C and 9D are views for describing a slab-typetwo-dimensional photonic crystal element provided with an air-bridgestructure in Example 4 according to the present invention, in whichFIGS. 9A and 9C are views for describing the case of producing awaveguide, and FIGS. 9B and 9D are views for describing the case ofproducing a resonator.

FIGS. 10A, 10B, 10C, 10D, 10E and 10F are views for describing a methodfor manufacturing a slab-type two-dimensional photonic crystal elementprovided with an air-bridge structure in Example 4 according to thepresent invention.

DESCRIPTION OF THE EMBODIMENTS

An optical element according to the present invention is configured tohave such a substrate and a photonic crystal layer provided thereon asto enable the difference of refractive indices between them to be largein order to confine light in the photonic crystal layer, by focusingattention on and making use of the fact that a semiconductor layerlocated under a photonic crystal layer can be selectively oxidized.

Specifically, the above described optical element is configured so as tohave the first layer and the second layer formed on the substrate,wherein the second layer includes pores and has a refractive-indexperiodically changing structure in which a refractive index periodicallychanges in an in-plane direction; and the first layer has an oxidizedregion having a lower refractive index than the refractive index of thesecond layer, in a lower part of the pores of the second layer.

The optical element having the above described configuration can preventthe photonic crystal layer from bending, when having the oxidized regionin the lower part of the photonic crystal including the pores, incomparison with the case of having no oxidized region (specifically, inthe case of an air-bridge structure).

Particularly, when the photonic crystal layer including the pores isprovided with a protective layer, the oxidized region shows a greateffect of preventing the photonic crystal layer from bending.

Furthermore, when a multilayered film including an active layer isprepared in the lower side (substrate side) of the first layer, theoxidized region can block or reduce the ingression of impurities intothe multilayered film from the outside.

In addition, when a light-emitting element is produced by using aslab-type two-dimensional photonic crystal element having aconfiguration of the present invention applied to it, the light-emittingelement has advantages described below.

Specifically, the light-emitting element can operate in a singletransverse mode with the whole post structure better than a conventionalsurface emitting laser which operates in a single transverse mode due toa current confinement structure; can emit a laser beam having a largerspot diameter than that of conventional single mode light; and therebycan provide a surface emitting laser having a larger optical output.

Embodiments

In the next place, examples according to the present invention will bedescribed.

EXAMPLE 1

In Example 1, a slab-type two-dimensional photonic crystal elementconfigured by applying the present invention will be described.

In the present example, a slab-type photonic crystal was formed on aGaAs substrate, which includes a selectively oxidized layer of an AlGaAslayer (containing 90% or more Al) used as a clad, and an AlGaAs layer(containing 70% or less Al) used as a core.

FIGS. 1A to 1C illustrate a configuration of the slab-typetwo-dimensional photonic crystal element according to the presentexample.

In FIGS. 1A to 1C, reference numeral 100 denotes the GaAs substrate,reference numeral 200 denotes the Al_(0.93)Ga_(0.07)As layer, referencenumeral 300 denotes the Al_(0.5)Ga_(0.5)As layer, reference numeral 400denotes a cylindrical hole and reference numeral 500 denotes an aluminumoxide layer.

As is illustrated in FIGS. 1A to 1C, a slab-type two-dimensionalphotonic crystal element produced in the present example includes a GaAssubstrate 100; an Al_(0.93)Ga_(0.07)As layer 200 with a thickness of 0.5μm epitaxially grown thereon; and an Al_(0.5)Ga_(0.5)As layer 300 with athickness of 0.2 μm epitaxially grown further thereon.

In the Al_(0.5)Ga_(0.5)As layer 300, cylindrical holes 400 penetratingthe Al_(0.5)Ga_(0.5)As layer are periodically arrayed into a triangularlattice form.

As illustrated in FIGS. 1A to 1C, in the present example, theAl_(0.5)Ga_(0.5)As layer 300 (refractive index of 3.5) has the periodicstructure (in photonic crystal containing cylindrical hole 400) formedtherein; contacts with air (refractive index of 1) on its top face; andcontacts with the aluminum oxide layer 500 (with refractive index of1.6) formed by selectively oxidizing the Al_(0.93)Ga_(0.07)As layer 200,in its lower face.

Accordingly, a periodic structure (photonic crystal) formed in theAl_(0.5)Ga_(0.5)As layer 300 (with refractive index of 3.5) functions asa core layer.

Thus, a slab-type two-dimensional photonic crystal element according tothe present example can acquire as large a difference of refractiveindices between the core layer and clads as about 1.9, though thedifference has been about 0.3 in a conventional semiconductor-stackedstructure.

Accordingly, the structure according to the present example enableslight to be strongly confined into the core layer, and enables awaveguide with a low optical loss as illustrated in FIGS. 2A and 2C anda resonator with a high Q-factor as illustrated in FIGS. 2B and 2D to beproduced.

In the next place, a method for manufacturing a slab-typetwo-dimensional photonic crystal element according to the presentexample will be described.

FIGS. 3A to 3E illustrate schematic views for describing the abovedescribed manufacturing method according to the present example.

In the views, reference numeral 100 denotes a GaAs substrate; referencenumeral 693 denotes an AlGaAs epitaxial layer (containing 93% or moreAl); reference numeral 650 denotes an AlGaAs epitaxial layer (containing50% or less Al); reference numeral 700 denotes a resist layer, referencenumeral 800 denotes a resist pattern; reference numeral 400 denotescylindrical holes; and reference numeral 500 denotes an aluminum oxidelayer.

At first as illustrated in FIG. 3A, an Al_(0.93)Ga_(0.07)As layer 693was grown into the thickness of 0.5 μm on a GaAs substrate 100 through abuffer layer by using an MOCVD apparatus, and then an Al_(0.5)Ga_(0.5)Aslayer 650 was grown into the thickness of 0.25 μm. Subsequently, as isillustrated in FIGS. 3B and C, a resist pattern 800 was formed on theAl_(0.5)Ga_(0.5)As layer with the use of an electron-beam lithographictechnique.

Subsequently, as illustrated in FIG. 3D, the Al_(0.5)Ga_(0.5)As layerwas dry-etched deeply until the Al_(0.93)Ga_(0.07)As layer 693 wasexposed, by using an ICP etching apparatus.

Subsequently, the resist was removed with an oxygen plasma ashingtechnique. Through the step, a slab-type two-dimensional photoniccrystal (containing cylindrical holes 400 arrayed in triangular lattice)was formed.

Subsequently, an aluminum oxide layer 500 was formed as illustrated inFIG. 3E, by charging thus treated substrate into an oxidizing apparatus(at 450° C., under water vapor atmosphere), and selectively oxidizingone part of the Al_(0.93)Ga_(0.07)As layer 693 through the cylindricalholes formed in the Al_(0.5)Ga_(0.5)As layer 650.

By the above described steps, such a stacked structure of asemiconductor core layer and an oxide film clad layer as to have alarger difference of refractive indices between a core layer and a cladlayer than a slab-type two-dimensional photonic crystal having asemiconductor-stacked structure can be easily produced. For instance, asemiconductor-multilayered film structure in one part of which aslab-type two-dimensional photonic crystal is formed can be easilyproduced.

In the present example, cylindrical holes were periodically arrayed in atriangular lattice form, but the form is not limited thereto. Thecylindrical holes may be arrayed into an arbitrary pattern such as atetragonal lattice and a honeycomb lattice. In addition, the shape ofthe hole is not limited to the cylindrical shape, but may be an ellipticcylindrical shape, a quadrangular prism shape or a triangular prismshape.

In addition, in the present example, an oxide film with a low refractiveindex was formed by selectively oxidizing AlGaAs (containing 90% or moreAl) in a system AlGaAs (containing 90% or more Al)/AlGaAs (containing70% or less Al), but it is not limited thereto. A material system whichcan provide a similar effect (selective oxidation), for instance, anAlN/GaN system may be used.

In addition, techniques (apparatuses) used for growth, lithography,etching and ashing as were shown in the present example are not limitedto the described techniques (apparatuses), but any technique (apparatus)is acceptable as long as the apparatus can provide the same effect.

EXAMPLE 2

In Example 2, a surface emitting laser will be described which employs aslab-type two-dimensional photonic crystal element according to thepresent invention.

FIG. 4 illustrates a configuration of the above described surfaceemitting laser according to the present example.

In FIG. 4, reference numeral 100 denotes a GaAs substrate, referencenumeral 920 denotes an n-typeAl_(0.93)Ga_(0.07)As/Al_(0.5)Ga_(0.5)As-DBR mirror layer, and referencenumeral 1020 denotes an n-type AlGaInP clad layer.

In addition, reference numeral 1100 denotes a GaInP/AlGaInP multiquantumwell active layer, reference numeral 1010 denotes a p-type AlGaInP cladlayer and reference numeral 210 denotes a p-type Al_(0.93)Ga_(0.07)Aslayer.

Furthermore, reference numeral 310 denotes a p-type Al_(0.5)Ga_(0.5)Aslayer, reference numeral 400 denotes a cylindrical hole, referencenumeral 500 denotes an aluminum oxide layer, reference numeral 1200denotes a silicon nitride layer, reference numeral 1300 denotes an anodeand reference numeral 1400 denotes a cathode.

In the present example, a two-dimensional photonic crystal surfaceemitting laser was produced so that nodes of a standing wave in aresonator could be formed at the center of resonator and each of theinterfaces of the active layer and the top and bottom mirrors (DBRmirror and slab-type two-dimensional photonic crystal mirror).

A multiquantum well active layer 1100 was formed on the resonator centerso that the gain could match.

In addition, in the present example, the reflecting mirror (DBR mirror)was formed of an Al_(0.93)Ga_(0.07)As/Al_(0.5)Ga_(0.5)Asmultilayered-film which was formed by alternately stacking a highrefractive-index medium with a thickness of a quarter wavelength and alow refractive-index medium with a thickness of a quarter wavelength,and was employed for the first mirror.

The slab-type two-dimensional photonic crystal mirror according to thepresent example was employed for the second mirror.

A slab-type two-dimensional photonic crystal mirror according to thepresent example has a periodic structure formed on the top layer of asemiconductor-multilayered film grown on a substrate, and the periodicstructure contacts with air on its top face. Specifically, the periodicstructure (in photonic crystal containing cylindrical holes arrayed intriangular lattice form) is formed on the Al_(0.5)Ga_(0.5)As layer (withrefractive index of 3.5) which is the top layer of the above describedsemiconductor-multilayered film, and contacts with the air (withrefractive index of 1) on its top face.

The periodic structure also contacts with an aluminum oxide layer (withrefractive index of 1.6) obtained by selectively oxidizing one part ofan Al_(0.93)Ga_(0.07)As layer, at its bottom face.

The slab-type two-dimensional photonic crystal functions as a mirror bymaking use of an effect referred to as Guided Resonance.

The Guided Resonance means a phenomenon that when light is incident onthe slab-type two-dimensional photonic crystal from a directionperpendicular to a slab face, a light having a predetermined frequencyis reflected back at an efficiency of nearly 100%. In other words, theGuided Resonance occurs when a mode propagating in the slab-typetwo-dimensional photonic crystal resonates with a particular radiationmode.

In the next place, a method for manufacturing a surface emitting laserwill be described which applies a slab-type two-dimensional photoniccrystal element according to the present example for a mirror.

FIGS. 5A to 5I illustrate schematic views for describing the method formanufacturing the above described surface emitting laser according tothe present example.

At first, as illustrated in FIG. 5A, the following respective layerswere grown on an n-type GaAs substrate through a buffer layer with theuse of a MOCVD apparatus: an n-typeAl_(0.93)Ga_(0.07)As/Al_(0.5)Ga_(0.5)As-DBR mirror layer, an n-typeAlGaInP clad layer, a GaInP/AlGaInP-MQW active layer and a p-typeAlGaInP clad layer; and then a p-type Al_(0.93)Ga_(0.07)As layer and ap-type Al_(0.5)Ga_(0.5)As layer.

In the present example, a semiconductor-multilayered film was composedof the above described respective layers which were thus grown.

Subsequently, a resist pattern was formed on an Al_(0.5)Ga_(0.5)As layeras illustrated in FIGS. 5B and C, by using an electron-beam lithographictechnique.

Subsequently, the Al_(0.5)Ga_(0.5)As layer was dry-etched deeply untilthe Al_(0.93)Ga_(0.07)As layer 920 was exposed as illustrated in FIG.5D, by using an ICP etching apparatus.

Subsequently, the resist was removed with an oxygen plasma ashingtechnique. Through the step, a slab-type two-dimensional photoniccrystal (containing cylindrical holes arrayed in triangular latticeform) was formed.

Subsequently, a resist pattern was formed on an Al_(0.5)Ga_(0.5)As layeras illustrated in FIG. 5E, by using a photolithographic technique.

Then, the layers on the n-typeAl_(0.93)Ga_(0.07)As/Al_(0.5)Ga_(0.5)As-DBR mirror layer was dry-etcheduntil the n-type Al_(0.93)Ga_(0.07)As/Al_(0.5)Ga_(0.5)As-DBR mirrorlayer was exposed as illustrated in FIG. 5F, by using an ICP etchingapparatus.

Subsequently, the resist was removed with an oxygen plasma ashingtechnique. Subsequently, a silicon nitride layer illustrated in FIG. 5Gwas formed by using a PECVD apparatus.

Then, the periodic structure (photonic crystal containing cylindricalholes) formed on the Al_(0.5)Ga_(0.5)As layer was exposed by using aphotolithographic technique, an RIE dry-etching technique and an oxygenplasma ashing technique.

Subsequently, an aluminum oxide layer was formed as illustrated in FIG.5H, by charging thus treated substrate into an oxidizing apparatus (at450° C., under water vapor atmosphere), and selectively oxidizing onepart of the Al_(0.93)Ga_(0.07)As layer through the cylindrical holesformed in the Al_(0.5)Ga_(0.5)As layer 310.

Subsequently, a Ti/Au anode was formed on the Al_(0.5)Ga_(0.5)As layerwith the use of a liftoff technique as illustrated in FIG. 5I. Inaddition, an AuGe/Au cathode was formed on the back face of the GaAssubstrate by using an electron-beam vapor deposition technique.

A surface emitting laser having a configuration of using a DBR mirrorand a slab-type two-dimensional photonic crystal mirror as a mirror forforming a vertical resonator was obtained by the above described steps.

The surface emitting laser configured so as to use a slab-typetwo-dimensional photonic crystal according to the present example as themirror can form a mirror having higher reflectivity than a conventionalsurface emitting laser, with a single layer.

The slab-type two-dimensional photonic crystal according to the presentexample can also provide the surface emitting laser having a lowerthreshold current for causing vibration than a conventional surfaceemitting laser, because the photonic crystal can decrease the resistanceof an element.

In the present example, cylindrical holes were periodically arrayed in atriangular lattice form, but the form is not limited thereto. Thecylindrical holes may be arrayed into an arbitrary pattern such as atetragonal lattice and a honeycomb lattice. In addition, the shape ofthe hole is not limited to the cylindrical shape, but may be an ellipticcylindrical shape, a quadrangular prism shape or a triangular prismshape.

In addition, in the present example, an oxide film with a low refractiveindex was formed by selectively oxidizing AlGaAs (containing 90% or moreAl) in a system AlGaAs (containing 90% or more Al)/AlGaAs (containing70% or less Al), but it is not limited thereto. A material system whichcan provide a similar effect (selective oxidation), for instance, anAlN/GaN system may be used.

In addition, techniques (apparatuses) used for growth, lithography,etching, ashing and vapor deposition as were illustrated in the presentexample are not limited to the described techniques (apparatuses), butany technique (apparatus) is acceptable as long as the apparatus canprovide the same effect.

In addition, in the present example, a resonator structure wasconfigured by an n-type Al_(0.93)Ga_(0.07)As/Al_(0.5)Ga_(0.5)As-DBRmirror 920 and a slab-type two-dimensional photonic crystal mirror, butis not limited to the configuration.

The resonator structure may employ the configuration of using thetwo-dimensional photonic crystal mirror, for instance, as a substitutefor the DBR mirror 920 illustrated in FIG. 4.

EXAMPLE 3

In Example 3, a surface emitting laser will be described which employs aslab-type two-dimensional photonic crystal element according to thepresent invention, but has a different form from that in Example 2.

FIG. 6 illustrates a configuration of the above described surfaceemitting laser according to the present example.

In FIG. 6, reference numeral 100 denotes a GaAs substrate, referencenumeral 920 denotes an n-typeAl_(0.93)Ga_(0.07)As/Al_(0.5)Ga_(0.5)As-DBR mirror layer, and referencenumeral 1020 denotes an n-type AlGaInP clad layer.

In addition, reference numeral 1100 denotes a GaInP/AlGaInP multiquantumwell active layer, reference numeral 1010 denotes a p-type AlGaInP cladlayer, reference numeral 210 denotes a p-type Al_(0.93)Ga_(0.07)Aslayer, and reference numeral 310 denotes a p-type Al_(0.5)Ga_(0.5)Aslayer.

Furthermore, reference numeral 400 denotes a cylindrical hole, referencenumeral 1600 denotes a defect, reference numeral 500 denotes an aluminumoxide layer, reference numeral 1200 denotes a silicon nitride layer,reference numeral 1300 denotes an anode and reference numeral 1400denotes a cathode.

In the present example, a two-dimensional photonic crystal surfaceemitting laser was produced so that nodes of a standing wave in aresonator could be formed at respective borders between a resonatorcenter and an active layer and the top and bottom mirrors (first andsecond mirrors).

A quantum well active layer 1100 was formed on the resonator center sothat the gain could match.

In addition, in the present example, a multilayered-film reflectingmirror was formed by alternately stacking a high refractive-index mediumwith a thickness of a quarter wavelength and a low refractive-indexmedium with a thickness of a quarter wavelength, and was employed forthe first mirror.

The slab-type two-dimensional photonic crystal mirror having a defectintroduced therein according to the present example was employed for thesecond mirror.

A slab-type two-dimensional photonic crystal mirror according to thepresent example has a periodic structure formed on the top layer of asemiconductor-multilayered film grown on a substrate, and the periodicstructure contacts with air.

Specifically, the periodic structure (in photonic crystal containingcylindrical holes arrayed in triangular lattice form) is formed on theAl_(0.5)Ga_(0.5)As layer (with refractive index of 3.5) which is the toplayer of the above described semiconductor-multilayered film formed onthe substrate, and contacts with air (with refractive index of 1) on itstop face.

The periodic structure also contacts with an aluminum oxide layer (withrefractive index of 1.6) obtained by selectively oxidizing one part ofan Al_(0.93)Ga_(0.07)As layer, at its bottom face.

The slab-type two-dimensional photonic crystal functions as a mirror bymaking use of an effect referred to as Guided Resonance.

The Guided Resonance means a phenomenon that when light is incident onthe slab-type two-dimensional photonic crystal from a directionperpendicular to a slab face, a light having a predetermined frequencyis reflected back at an efficiency of nearly 100%.

In addition, the periodic structure (photonic crystal) formed in thepresent example has such a site (defect) as to disturb the periodicstructure, introduced in its one part.

The surface emitting laser having the defect formed in thetwo-dimensional photonic crystal can make incident light on atwo-dimensional photonic crystal mirror resonate in a wider range in acrystal plane, and can enlarge a spot size of an outgoing beam.

The surface emitting laser can also control an oscillation mode and apolarization mode by changing the shape of the defect.

The surface emitting laser having the configuration can also unify modesinto a single mode, because a level is formed in a photonic band by theintroduced defect, and incident light on the two-dimensional photoniccrystal mirror resonate only in the mode due to the level by the defectin an in-plane direction.

The light which has been united into the single mode is emitted throughthe defective site in a vertical direction of an incident light side,and resonates between the two top and bottom mirrors (at least one ofwhich a slab-type two-dimensional photonic crystal mirror having thedefect) which are formed so as to sandwich an active layer. Thus, thesurface emitting laser finally emits the coherent light.

When the spatially localized single-mode lights are emitted, they areunited to form a single mode light with a large spot diameter.

In the next place, a method for manufacturing a surface emitting laserwill be described which applies a slab-type two-dimensional photoniccrystal having a defect introduced therein according to the presentexample for a mirror.

FIGS. 7A to 7I illustrate schematic views for describing the abovedescribed manufacturing method according to the present example.

At first, as illustrated in FIG. 7A, the following respective layerswere grown on an n-type GaAs substrate through a buffer layer with theuse of a MOCVD apparatus: an n-typeAl_(0.93)Ga_(0.07)As/Al_(0.5)Ga_(0.5)As-DBR mirror layer, an n-typeAlGaInP clad layer, a GaInP/AlGaInP-MQW active layer and a p-typeAlGaInP clad layer; and then a p-type Al_(0.93)Ga_(0.07)As layer and ap-type Al_(0.5)Ga_(0.5)As layer.

In the present example, a semiconductor-multilayered film was composedof the above described respective layers which were thus grown.

Subsequently, a resist pattern was formed on an Al_(0.5)Ga_(0.5)As layeras illustrated in FIGS. 7B and C, by using an electron-beam lithographictechnique.

Subsequently, the Al_(0.5)Ga_(0.5)As layer was dry-etched deeply untilthe Al_(0.93)Ga_(0.07)As layer was exposed as illustrated in FIG. 7D, byusing an ICP etching apparatus.

Subsequently, the resist was removed with an oxygen plasma ashingtechnique.

Through the step, a slab-type two-dimensional photonic crystal(containing cylindrical holes arrayed in triangular lattice form) wasformed.

Subsequently, a resist pattern was formed on an Al_(0.5)Ga_(0.5)As layeras illustrated in FIG. 7E, by using a photolithographic technique.

Then, the layers on the n-typeAl_(0.93)Ga_(0.07)As/Al_(0.5)Ga_(0.5)As-DBR mirror layer was dry-etcheduntil the n-type Al_(0.93)Ga_(0.07)As/Al_(0.5)Ga_(0.5)As-DBR mirrorlayer was exposed as illustrated in FIG. 7F, by using an ICP etchingapparatus.

Subsequently, the resist was removed with an oxygen plasma ashingtechnique. Subsequently, a silicon nitride layer illustrated in FIG. 7Gwas formed by using a PECVD apparatus.

Then, the periodic structure (photonic crystal containing cylindricalholes) formed on the Al_(0.5)Ga_(0.5)As layer was exposed by using aphotolithographic technique, an RIE dry-etching technique and an oxygenplasma ashing technique.

Subsequently, an aluminum oxide layer was formed as illustrated in FIG.7H, by charging thus treated substrate into an oxidizing apparatus (at450° C., under water vapor atmosphere), and selectively oxidizing onepart of the Al_(0.93)Ga_(0.07)As layer through the cylindrical holesformed in the Al_(0.5)Ga_(0.5)As layer.

Subsequently, a Ti/Au anode was formed on the Al_(0.5)Ga_(0.5)As layerwith the use of a liftoff technique as illustrated in FIG. 7I. Inaddition, an AuGe/Au cathode was formed on the back face of the GaAssubstrate by using an electron-beam vapor deposition technique.

A surface emitting laser having a configuration of using a DBR mirrorand a slab-type two-dimensional photonic crystal mirror having a defectintroduced therein as a mirror for forming a vertical resonator wasobtained by the above described steps.

The surface emitting laser according to the present invention configuredso as to use a two-dimensional photonic crystal according to the presentexample can operate in a single transverse mode due to the whole poststructure better than a conventional surface emitting laser whichoperates in a single transverse mode due to a current confinementstructure.

In addition, the surface emitting laser according to the present examplecan emit a laser beam having a larger spot diameter than that of aconventional single-mode light, and consequently can emit the laser beamwith a high optical output.

The two-dimensional photonic crystal surface emitting laser having apost with a diameter of 20 μm produced with a method according to thepresent example emitted a light having a spot diameter of 15 μm (nearfield pattern).

The surface emitting laser which uses a slab-type two-dimensionalphotonic crystal having the configuration according to the presentexample as the mirror can also form a mirror having higher reflectivitythan a conventional surface emitting laser, with a single layer, andaccordingly can decrease the resistance of the device. As a result, thesurface emitting laser can provide a lower threshold current for causingvibration than a conventional surface emitting laser.

In the present example, cylindrical holes were periodically arrayed in atriangular lattice form, but the form is not limited thereto. Thecylindrical holes may be arrayed into an arbitrary pattern such as atetragonal lattice and a honeycomb lattice. In addition, the shape ofthe hole is not limited to the cylindrical shape, but may be an ellipticcylindrical shape, a quadrangular prism shape or a triangular prismshape.

In addition, in the present example, an oxide film with a low refractiveindex was formed by selectively oxidizing AlGaAs (containing 90% or moreAl) in a system AlGaAs (containing 90% or more Al)/AlGaAs (containing70% or less Al), but it is not limited thereto. A material system whichcan provide a similar effect (selective oxidation), for instance, anAlN/GaN system may be used.

In addition, techniques (apparatuses) used for growth, lithography,etching, ashing and vapor deposition as were illustrated in the presentexample are not limited to the described techniques (apparatuses), butany technique (apparatus) is acceptable as long as the apparatus canprovide the same effect.

In addition, in the present example, a resonator structure wasconfigured by an n-type Al_(0.93)Ga_(0.07)As/Al_(0.5)Ga_(0.5)As-DBRmirror 920 and a slab-type two-dimensional photonic crystal mirrorhaving a defect introduced therein, but is not limited to theconfiguration.

The resonator structure may employ the configuration of using atwo-dimensional photonic crystal mirror or the two-dimensional photoniccrystal mirror having the defect (not shown), for instance, as asubstitute for the DBR mirror 920 illustrated in FIG. 6.

EXAMPLE 4

In Example 4, a slab-type two-dimensional photonic crystal elementhaving an air-bridge structure configured by applying the presentinvention will be described.

In the present example, one part of an AlGaAs layer (containing 90% ormore Al) according to the present invention is selectively oxidized, andonly the oxidized layer is further selectively etched. By the abovesteps, an air-bridge-type slab-type photonic crystal which employs anAlGaAs layer (containing 70% or less Al) for a core is formed on a GaAssubstrate.

FIGS. 8A to 8C illustrate a configuration of the slab-typetwo-dimensional photonic crystal element provided with the air-bridgestructure according to the present example.

In FIGS. 8A to 8C, reference numeral 100 denotes the GaAs substrate,reference numeral 200 denotes an Al_(0.93)Ga_(0.07)As layer, referencenumeral 300 denotes an Al_(0.5)Ga_(0.5)As layer, reference numeral 400denotes a cylindrical hole and reference numeral 1500 denotes theair-bridge structure.

As is illustrated in FIGS. 8A to 8C, the slab-type two-dimensionalcrystal element provided with the air-bridge structure according to thepresent example includes: a GaAs substrate 100; an Al_(0.93)Ga_(0.07)Aslayer 200 which is epitaxially grown into 0.5 μm on the GaAs substrate100; and an Al_(0.5)Ga_(0.5)As layer 300 which is epitaxially grown into0.2 μm further thereon. The Al_(0.5)Ga_(0.5)As layer 300 has cylindricalholes 400 which penetrate the Al_(0.5)Ga_(0.5)As layer and areperiodically arrayed into a triangular lattice form.

As illustrated in FIGS. 8A to 8C, in the present example, anAl_(0.5)Ga_(0.5)As layer 300 (with refractive index of 3.5) has aperiodic structure (of cylindrical holes 400 in photonic crystal) formedtherein and contacts with air (with refractive index of 1) on its topface.

The Al_(0.5)Ga_(0.5)As layer 300 also forms an air-bridge structure 1500that contacts with the atmospheric layer (with refractive index of 1) atits bottom face, which is formed by selectively removing an aluminumoxide layer (having refractive index of 1.6 but not shown) obtained byselectively oxidizing the Al_(0.93)Ga_(0.07)As layer 200. Thus, theslab-type two-dimensional photonic crystal element having aconfiguration according to the present example can acquire as large adifference of refractive indices between the core layer and clads asabout 2.5, though the difference has been about 0.3 in a conventionalsemiconductor-stacked structure.

Accordingly, the configuration according to the present example enableslight to be strongly confined into the core layer, and enables awaveguide with a low optical loss as illustrated in FIG. 9A and aresonator with a high Q-factor as illustrated in FIG. 9B to be produced.

In the next place, a method for manufacturing a slab-typetwo-dimensional photonic crystal element provided with the air-bridgestructure according to the present example will be described.

FIGS. 10A to 10F illustrate schematic views for describing the abovedescribed manufacturing method according to the present example.

In FIGS. 10A to 10F, reference numeral 100 denotes a GaAs substrate,reference numeral 693 denotes an AlGaAs epitaxial layer (containing 93%or more Al), and reference numeral 650 denotes an AlGaAs epitaxial layer(containing 50% or less Al).

In addition, reference numeral 700 denotes a resist layer, referencenumeral 800 denotes a resist pattern, reference numeral 400 denotes acylindrical hole, reference numeral 500 denotes an aluminum oxide layerand reference numeral 1500 denotes an air-bridge structure.

At first, as illustrated in FIG. 10A, an Al_(0.93)Ga_(0.07)As layer 693was grown into the thickness of 0.5 μm on a GaAs substrate 100 through abuffer layer by using an MOCVD apparatus, and then an Al_(0.5)Ga_(0.5)Aslayer 650 was grown into the thickness of 0.25 μm. Subsequently, as isillustrated in FIGS. 10B and C, a resist pattern 700 was formed on theAl_(0.5)Ga_(0.5)As layer with the use of an electron-beam lithographictechnique.

Subsequently, the Al_(0.5)Ga_(0.5)As layer was dry-etched deeply untilthe Al_(0.93)Ga_(0.07)As layer was exposed as illustrated in FIG. 10D,by using an ICP etching apparatus.

Subsequently, the resist was removed with an oxygen plasma ashingtechnique.

Through the step, a slab-type two-dimensional photonic crystal(containing cylindrical holes arrayed in triangular lattice form) wasformed.

Subsequently, an aluminum oxide layer 500 was formed as illustrated inFIG. 10E, by charging thus treated substrate into an oxidizing apparatus(at 450° C., under water vapor atmosphere), and selectively oxidizingone part of the Al_(0.93)Ga_(0.07)As layer through the cylindrical holesformed in the Al_(0.5)Ga_(0.5)As layer.

Then, an air-bridge structure 1500 was formed as illustrated in FIG.10F, by selectively etching the aluminum oxide layer 500 with the use ofa buffered fluoric acid solution to form the atmospheric layer.

By the above described steps, such a stacked structure of asemiconductor core layer and an air clad layer as to have a largerdifference of refractive indices between the core layer and the cladlayer than a slab-type two-dimensional photonic crystal having asemiconductor-stacked structure can be easily produced.

For instance, a semiconductor-multilayered film structure in one part ofwhich a slab-type two-dimensional photonic crystal is formed can beeasily produced.

In the present example, cylindrical holes were periodically arrayed in atriangular lattice form, but the form is not limited thereto. Thecylindrical holes may be arrayed into an arbitrary pattern such as atetragonal lattice and a honeycomb lattice. In addition, the shape ofthe hole is not limited to the cylindrical shape, but may be an ellipticcylindrical shape, a quadrangular prism shape or a triangular prismshape.

In addition, in the present example, an oxide film with a low refractiveindex was formed by selectively oxidizing AlGaAs (containing 90% or moreAl) in a system AlGaAs (containing 90% or more Al)/AlGaAs (containing70% or less Al), but it is not limited thereto. A material system whichcan provide a similar effect (selective oxidation), for instance, anAlN/GaN system may be used.

In addition, techniques (apparatuses) used for growth, lithography,etching and ashing as were illustrated in the present example are notlimited to the described techniques (apparatuses), but any technique(apparatus) is acceptable as long as the apparatus can provide the sameeffect.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2006-051945, filed Feb. 28, 2006, which is hereby incorporated byreference herein in its entirety.

1. An optical element comprising a first layer and a second layer formedon a substrate, wherein the second layer includes pores and has arefractive-index periodically changing structure in which a refractiveindex changes periodically in an in-plane direction, wherein the firstlayer has an oxidized region with a lower refractive index than therefractive index of the second layer, in a lower side of the pores ofthe second layer, and wherein the first layer includes the oxidizedregion and an unoxidized region.
 2. The optical element according toclaim 1, wherein the first layer includes Al.
 3. The optical elementaccording to claim 1, wherein a difference in the refractive indices ofthe oxidized region and the second layer is 1.0 or more.
 4. The opticalelement according to claim 1, wherein the first layer and the secondlayer include Al, Ga, and As.
 5. The optical element according to claim1, wherein the optical element is incorporated in a semiconductor laserdevice, and wherein the optical element is used as a mirror in a surfaceemitting laser.